This study is an international, multicenter, pilot, open-label evaluation of the safety, tolerability, and efficacy of 534U87 add-on treatment of partial seizures. The safety and tolerability of 534U87 will be assessed by adverse event reporting, clinical laboratory assessments, physical and neurological examinations, electrocardiograms, vital signs, and body height and weight. Possible drug interactions with other concomitant AEDs will also be assessed during this trial. Preliminary evidence of efficacy for this study will be based on measurements of seizure frequency, including the proportion of subjects with 50% reduction in seizure frequency. For the purposes of this study, partial seizures are defined as: objective (behavioral or motor manifestations obvious to the patient and/or his/her family/caregiver/ companion) simple partial (Type A), complex partial (Type B), and secondarily generalized tonic-clonic, clonic or tonic (Type C) seizures. The study will enroll sufficient subjects to yield 100 "completer" subjects, i.e. subjects who complete the study through the Maintenance Phase (expected enrollment <150 subjects).